Chapter 34. Images. Copyright 2014 John Wiley & Sons, Inc. All rights reserved.

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1 Chapter 34 Images Copyright 2014 John Wiley & Sons, Inc. All rights

2 Images Images and and Plane Plane An image is a reproduceon of an object via light. If the image can form on a surface, it is a real image and can exist even if no observer is present. If the image requires the visual system of an observer, it is a virtual image. Here are some common examples of virtual image. (a) A ray from a low seceon of the sky refracts through air that is heated by a road (without reaching the road). An observer who intercepts the light perceives it to be from a pool of water on the road. (b) Bending (exaggerated) of a light ray descending across an imaginary boundary from warm air to warmer air. (c) ShiQing of wavefronts and associated bending of a ray, which occur because the lower ends of wavefronts move faster in warmer air. (d) Bending of a ray ascending across an imaginary boundary to warm air from warmer air.

3 Images Images and and Plane Plane As shown in figure (a), a plane (flat) mirror can form a virtual image of a light source (said to be the object, O) by redireceng light rays emerging from the source. The image can be seen where backward extensions of reflected rays pass through one another. The object s distance p from the mirror is related to the (apparent) image distance i from the mirror by (a) Object distance p is a posieve quanety. Image distance i for a virtual image is a negaeve quanety. Only rays that are fairly close together can enter the eye aqer refleceon at a mirror. For the eye posieon shown in Fig. (b), only a small poreon of the mirror near point a (a poreon smaller than the pupil of the eye) is useful in forming the image. (b)

4 Spherical Spherical A spherical mirror is in the shape of a small seceon of a spherical surface and can be concave (the radius of curvature r is a posieve quanety), convex (r is a negaeve quanety), or plane (flat, r is infinite). We make a concave mirror by curving the mirror s surface so it is concave ( caved in to the object) as in Fig. (b). We can make a convex mirror by curving a plane mirror so its surface is convex ( flexed out ) as in Fig.(c). Curving the surface in this way (1) moves the center of curvature C to behind the mirror and (2) increases the field of view. It also (3) moves the image of the object closer to the mirror and (4) shrinks it. These iterated characterisecs are the exact opposite for concave mirror.

5 Spherical Spherical If parallel rays are sent into a (spherical) concave mirror parallel to the central axis, the reflected rays pass through a common point (a real focus F ) at a distance f (a posieve quanety) from the mirror (figure a). If they are sent toward a (spherical) convex mirror, backward extensions of the reflected rays pass through a common point (a virtual focus F ) at a distance f (a negaeve quanety) from the mirror (figure b). For mirrors of both types, the focal length f is related to the radius of curvature r of the mirror by where r (and f) is posieve for a concave mirror and negaeve for a convex mirror.

6 Spherical Spherical (a) An object O inside the focal point of a concave mirror, and its virtual image I. (b) The object at the focal point F. (c) The object outside the focal point, and its real image I. A concave mirror can form a real image (if the object is outside the focal point) or a virtual image (if the object is inside the focal point). A convex mirror can form only a virtual image. The mirror equaeon relates an object distance p, the mirror s focal length f and radius of curvature r, and the image distance i: The magnitude of the lateral magnificaeon m of an object is the raeo of the image height h to object height h,

7 Spherical Spherical Loca7ng Images by Drawing Rays 1. A ray that is inieally parallel to the central axis reflects through the focal point F (ray 1 in Fig. a). 2. A ray that reflects from the mirror aqer passing through the focal point emerges parallel to the central axis (Fig. a). 3. A ray that reflects from the mirror aqer passing through the center of curvature C returns along itself (ray 3 in Fig. b). 4. A ray that reflects from the mirror at point c is reflected symmetrically about that axis (ray 4 in Fig. b). The image of the point is at the interseceon of the two special rays you choose. The image of the object can then be found by locaeng the images of two or more of its off-axis points (say, the point most off axis) and then sketching in the rest of the image. You need to modify the descripeons of the rays slightly to apply them to convex mirrors, as in Figs. c and d.

8 Thin Thin Lenses Lenses Forming a Focus. Figure (a) shows a thin lens with convex refraceng surfaces, or sides. When rays that are parallel to the central axis of the lens are sent through the lens, they refract twice, as is shown enlarged in Fig.(b). This double refraceon causes the rays to converge and pass through a common point F 2 at a distance f from the center of the lens. Hence, this lens is a converging lens; further, a real focal point (or focus) exists at F 2 (because the rays really do pass through it), and the associated focal length is f. When rays parallel to the central axis are sent in the opposite direceon through the lens, we find another real focal point at F 1 on the other side of the lens. For a thin lens, these two focal points are equidistant from the lens.

9 Thin Thin Lenses Lenses Forming a Focus. Figure (c) shows a thin lens with concave sides. When rays that are parallel to the central axis of the lens are sent through this lens, they refract twice, as is shown enlarged in Fig. (d); these rays diverge, never passing through any common point, and so this lens is a diverging lens. However, extensions of the rays do pass through a common point F 2 at a distance f from the center of the lens. Hence, the lens has a virtual focal point at F 2. (If your eye intercepts some of the diverging rays, you perceive a bright spot to be at F 2, as if it is the source of the light.) Another virtual focus exists on the opposite side of the lens at F 1, symmetrically placed if the lens is thin. Because the focal points of a diverging lens are virtual, we take the focal length f to be negaeve.

10 Thin Thin Lenses Lenses Loca7ng Images of Extended Objects by Drawing Rays 1. A ray that is inieally parallel to the central axis of the lens will pass through focal point F 2 (ray 1 in Fig. a). 2. A ray that inieally passes through focal point F 1 will emerge from the lens parallel to the central axis (ray 2 in Fig. a). 3. A ray that is inieally directed toward the center of the lens will emerge from the lens with no change in its direceon (ray 3 in Fig. a) because the ray encounters the two sides of the lens where they are almost parallel. Figure b shows how the extensions of the three special rays can be used to locate the image of an object placed inside focal point F 1 of a converging lens. Note that the descripeon of ray 2 requires modificaeon (it is now a ray whose backward extension passes through F 1 ).You need to modify the descripeons of rays 1 and 2 to use them to locate an image placed (anywhere) in front of a diverging lens. In Fig. c, for example, we find the point where ray 3 intersects the backward extensions of rays 1 and 2.

11 Chapter 35 Interference Copyright 2014 John Wiley & Sons, Inc. All rights

12 Light Light as as a a Wave Wave The three-dimensional transmission of waves, including light, may oqen be predicted by Huygens principle, which states that Figure 1 shows the propagaeon of a plane wave in vacuum, as portrayed by Huygens principle. Figure 1 The refraceon of a plane wave at an air glass interface, as portrayed by Huygens principle. The wavelength in glass is smaller than that in air. For simplicity, the reflected wave is not shown. Parts (a) through (c) represent three successive stages of the refraceon.

13 Light Light as as a a Wave Wave The refraceon of a plane wave at an air glass interface, as portrayed by Huygens principle. The wavelength in glass is smaller than that in air. For simplicity, the reflected wave is not shown. Parts (a) through (c) represent three successive stages of the refraceon. The law of refraceon can be derived from Huygens principle by assuming that the index of refraceon of any medium is n = c/v, in which v is the speed of light in the medium and c is the speed of light in vacuum. The wavelength λ n of light in a medium depends on the index of refraceon n of the medium: Because of this dependency, the phase difference where between λ is the two wavelength waves can of change vacuum if they pass through different materials with different indexes of refraceon.

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16 Young s Young s Interference Interference The flaring is consistent with the spreading of wavelets in the Huygens construceon. Diffrac7on occurs for waves of all types, not just light waves. Figure below shows waves passing through a slit flares. Figure (a) shows the situaeon schemaecally for an incident plane wave of wavelength λ encountering a slit that has width a = 6.0 λ and extends into and out of the page. The part of the wave that passes through the slit flares out on the far side. Figures (b) (with a = 3.0 λ ) and (c) (a = 1.5λ ) illustrate the main feature of diffraceon: the narrower the slit, the greater the diffraceon.

17 Young s Young s Interference Interference Figure gives the basic arrangement of Young s experiment. Light from a distant monochromaec source illuminates slit S 0 in screen A. The emerging light then spreads via diffraceon to illuminate two slits S 1 and S 2 in screen B. DiffracEon of the light by these two slits sends overlapping circular waves into the region beyond screen B, where the waves from one slit interfere with the waves from the other slit. A photograph of the interference pafern produced by the arrangement shown in the figure(right), but with short slits. (The photograph is a front view of part of screen C of figure on leq.) The alternaeng maxima and minima are called interference fringes (because they resemble the decoraeve fringe someemes used on clothing and rugs).

18 Young s Young s Interference Interference (a) Waves from slits S 1 and S 2 (which extend into and out of the page) combine at P, an arbitrary point on screen C at distance y from the central axis. The angle θ serves as a convenient locator for P. (b) For D >> d, we can approximate rays r 1 and r 2 as being parallel, at angle θ to the central axis. The condieons for maximum and minimum intensity are

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23 Interference Interference and and Double-Slit Double-Slit Intensity Intensity If two light waves that meet at a point are to interfere percepebly, both must have the same wavelength and the phase difference between them must remain constant with Eme; that is, the waves must be coherent. As shown in figure, in Young s interference experiment, two waves, each with intensity I 0, yield a resultant wave of intensity I at the viewing screen, with A plot of equaeon below, showing the intensity of a double-slit interference pafern as a funceon of the phase difference between the waves when they arrive from the two slits. I 0 is the (uniform) intensity that would appear on the screen if one slit were covered. The average intensity of the fringe pafern is 2I 0, and the maximum intensity (for coherent light) is 4I 0. where